Search for Cosmic Neutrino Point Sources

Introduction

The strength of neutrino astronomy lies in the fact that neutrinos are unlike photons not subject to adsorption by matter or radiation and do not get scattered by magnetic fields, such as charged cosmic ray particles. It therefore provides a possibility to investigate cosmic objects at an energy range where they cannot be observed by other means.
One of the main areas of investigation is the search for steady point sources. Assuming generic emission spectra and taking advantage of the large sky coverage and near-continuous exposition of the AMANDA/IceCube array, this type of analysis has excellent potential for discovery of a signal.

Motivation

One of the principal reasons to search for neutrino point sources is the differentiation between hadronic and leptonic models in cosmic accelerators, such as active galactic nuclei (AGN). Many such objects are known sources of Very High Energy (> tens of GeV) gamma rays, but the exact mechanisms leading to those emissions remain unclear.
A common feature of AGN emission spectra is the presence of two distinct peaks, one in the X-ray region, presumably caused by synchrotron emission, and the other in the range from several MeV up to hundreds of GeV, depending on the individual object. Investigating the source of the second peak is one of the main goals of contemporary astrophysics and one to which neutrino astrophysics can make an important contribution.
Emission models can be roughly divided into two classes: hadronic and leptonic. As the name suggests, leptonic models involve only leptons and photons, typically consisting of the production of high energy gamma rays through the interaction of an electron with an ambient photon ("Inverse Compton Scattering"). The competing model - hadronic emission - assumes production of gamma rays through the decay of mesons produced in interactions of high energy protons with photons or other particles present in the jet. Since neutrinos will be produced in hadronic interactions, but not in leptonic ones, detection or exclusion of neutrino emissions from known VHE gamma sources allows to decide in favor of one model or the other.

Potential Sources

A variety of possible cosmic neutrino sources have been proposed using theoretical considerations. Other than the above-mentioned AGNs, there are various galactic and extragalactic objects which should be considered. One important criterion is the presence of high energy gamma rays. Any source of those can be considered a potential source of high energy neutrinos as well. The most important types are:

• Microquasars: The emission mechanism is similar to that of AGN, the difference being that microquasar jets originate from stellar-sized black holes in our own galaxy.

• Supernova Remnants (SNR): Gamma rays have been detected which are assumed to be related to Fermi acceleration of electrons in the shock front of the ejected shell. Some models also require a hadronic component, thus leading to neutrino emissions.

• Plerionic Pulsars: This type of object, consisting of a relatively young pulsar surrounded by ejected stellar matter, is well established as a VHE gamma ray source, the most notable example being the Crab nebula. Neutrino production models assume acceleration of protons by the magnetic field of the rapidly rotating neutron star in the center.

• Unknown VHE sources: There are several sources of VHE gamma rays in the galactic plane that cannot be unambiguously associated with counterparts in other wavelengths. Detecting neutrinos from those sources would provide an important clue for their identification.

Results

Since completion of its first stage ("B10") in 1997, the AMANDA array has been the most sensitive instrument for neutrino point source searches available. Several studies have already been completed, unfortunately all without discovering a signal. The latest such analysis was performed at DESY in Zeuthen using data taken with AMANDA during the years 2000 to 2003. The result, in the form of a sky plot showing excess of signal over background, is shown in the picture on the right. Using statistical analysis, it was confirmed that the probability distribution is fully consisted with random background fluctuations.
Currently, work is under way at Brussels to complete reanalysis of older (97-99) data using advanced reconstruction and calibration methods. Fully reanalyzed, the amount of statistics is expected to correspond to one year of the fully completed AMANDA array after 2000.
In the future, data from IceCube will allow searches with sensitivities improved by at least one order of magnitude, which should be sufficient to confirm or reject most currently existing neutrino emission models.